What is a suitable term to complete this sentence: The student's __________ understanding of the topic impressed the professor.

The term "comprehensive" is fitting in this context because it conveys a sense of thoroughness and depth in the student's understanding of the topic. When someone possesses a comprehensive understanding, they have a well-rounded grasp of all aspects and details related to the subject matter, which would certainly impress a professor. This term suggests that the student not only knows the fundamental concepts but also understands the intricacies and complexities involved, demonstrating a high level of knowledge and engagement with the topic.

In contrast, terms like "casual," "superficial," and "limited" imply a lack of depth or seriousness in the student's understanding, which would not be impressive to a professor. A casual understanding suggests a relaxed or informal approach that may overlook important details, a superficial understanding indicates a shallow grasp that misses key elements, and a limited understanding points to a narrow or restricted knowledge base. All of these terms would suggest shortcomings rather than a strong, admirable grasp of the material.